I know that Adobe Premiere does this, because I have the app and have done it before. However, Premiere is not a cheap program, and I wouldn't recommend buying it just to rip audio from a video file. I'll take a look around and let you know if I find something else.
VirtualDub... of course! I've used this program for years and never even noticed that you could copy audio. Definitely a good choice there. It will copy to .wav, and then all you have to do is encode it into whatever format you wish.
AVI is just two files (one audio and one video) combined, or "multiplexed", together. They're not mixed like paint, but more like they're intertwined like two cords of a rope. And just like a rope, those two pieces can be separated without having to re-encode either one.